In a steel material, such as a line pipe, which needs to have hydrogen induced cracking resistance, addition of Ca so as to cause Ca to react with S in the steel to form CaS is effective at making MnS that causes hydrogen induced cracking harmless. However, when Ca is added, the added Ca reacts with Al2O3, which is a deoxidation product, and CaO—Al2O3 inclusions are thereby generated. If the amount of Ca is insufficient, some part of S in the steel remains unreacted, causing generation of MnS. If the amount of Ca is excessively large, oxide with high CaO content is generated. Both the above MnS and oxide cause deterioration of hydrogen induced cracking resistance. Therefore, to improve the hydrogen induced cracking resistance, it is advantageous to add Ca such that the composition of the inclusions is controlled appropriately.
If the amount of Ca added is excessively large, CaO inclusions may cause HIC (Hydrogen Induced Cracking) to occur. Therefore, in aluminum killed steel, it is advantageous that the required amount of Ca be added according to the amount of Al2O3 present in molten steel before addition of Ca to thereby control the composition of the inclusions so that the inclusions are made harmless. Patent Literature 1 discloses a method of optimally controlling the amount of Ca added. In this method, after completion of secondary refining, the total oxygen content (T.[O]) in molten steel is analyzed, and Ca is added to the molten steel in an amount determined on the basis of the analysis results immediately before the start of pouring of the molten steel into a tundish.